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64                                 CHEMICAL ENGINEERING
The following values are given for expected mean effective pressures (pm) under good conditions:
Fuel	Compression, pounds per square inch, absolute :						
	70	80	90	100	120	140	160
Natural gas, water gas, coal gas .	62-82	67-87	70-97	70-100 55-80	83-103 62-87 60-85	65-90 60-90	75-90 70-95
Efficiency.—The maximum ideal efficiency (Otto cycle) is
- (¥5)
where PC = compression pressure, pounds per square inch, absolute. The probable actual efficiency lies between 0.45-Er to OMEi for most fuels, and from 0.40J5r to 0.50j& for blast-furnace gas. The heat consumption per indicated horsepower per minute is therefore 42.42/0.5^ = 84.84/^j B.t.u., approximately. The weight of gas consumed per indicated horsepower per minute is S4:M/BE2 lb., where B = low-heat value of gas, B.t.u. per pound.
£ 16 I 12
*""   20 30  40   50   60 10  80 90  100 110  120  130 140 150 Per Cent of Rated, B. Hp
FIG. 28.—Efficiency curves of internal-combustion engines.1
Efficiency varies chiefly with the compression and compression is limited by the danger of preignition. Gases high in hydrogen content ignite readily and must be used at low compression. Hence, the gases of high heat value are in general those least useful for engines.
The variation of efficiency with load depends on the method of governing. The hit-and-miss method of governing is probably most economical of fuel but gives irregular action and is used only on small engines. Variation of quantity of combustible mixture varies the compression with the load and hence decreases efficiency seriously at light loads. Variation of mixture strength leads to less change of compression but introduces other losses at light loads, due to impaired combustion. Misfires may be produced. The diagram shows in all cases maximum efficiency at maximum load: a characteristic of the gas engine, which has no overload capacity beyond its output of best economy. All of these engines were governed by adjustment
1 Curve 1, Deutz single cylinder, illuminating gas; Curve 2, Westinghouse, 3-cylinder vertical, natural gas; Curve 3, Dentz, producer gas; Curve 4, Gtildner, single cylinder, producer gas; Curve 5, Nurnberg, blast-furnace gas. (Cf. Marks, Mechanical Engineers' Handbook, 1st. ed., p. 1029.)